reNX: Fix small issue where nodes are parsed multiple times. Target .NET 4.

CLR 4 shows large improvements in.. basically everything. No point not targetting it. The code will still support .NET 3.5. In fact, the code can run on .NET 2.0, if NXFile.ResolvePath is rewritten without LINQ.

Signed-off-by: angelsl <hidingfromhidden@gmail.com>